A good defense is a best offense? A good offense is a best defense? Why not both?
This is the situation involving an enemy charging at you and you then defensively put a nasty, pointy surprise in front of you. Result: The enemy runs onto the pointy end, and there may be a moment where the hero and their enemy stare at each other before they reveal that the foe has just killed themselves. By waiting until the last moment to reveal the victor of the battle - when the hero and the enemy are close enough to touch - the situation comes to an exciting and dramatic conclusion.
This trope does not apply to every impalement. It only applies if the enemy is impaled by his own momentum without the defender needing to move from their positions.
Sub-trope of Impaled with Extreme Prejudice, Inertia Is a Cruel Mistress, and when the one impaled is a villain, Self-Disposing Villain. Also a subtrope of Deadly Dodging, for when the dodging results in the enemy impaling himself on something. See also Bullfight Boss.
As this is often a Death Trope, all Spoilers will be unmarked ahead. Beware.
One scene in Fist of the North Star shows the villain's fortifications, including sharpened trees thicker than a man's arm. Later on the downtrodden villagers storm the villain's base, and one guy is shown to have somehow impaled himself at least three feet on one of the stakes.
Berserk. Casca scored her first kill this way. This happened when the nobleman, who had supposedly taken her under his wing, tried to rape her. Griffith offered his sword to Casca so she had something to defend herself with, but since at the time Casca was just an ordinary peasant girl, she couldn't have killed the nobleman without this trope. Casca even lampshades this herself:
I don't know whether I stabbed him or if he fell onto the blade. It was my first kill. I was so scared I couldn't cry and couldn't move. I just gripped the bloody sword tightly.
In the movie Brother Bear, Kenai is fighting a bear and gets knocked on the ground. When it charges at him, he grabs his spear. Rather than showing what happens, it shows the mountain from a distance and the viewer hears the bear roar one last time, and then it shows that Kenai has survived.
In the film of The Hobbit, a Warg runs at Bilbo and he kills it by holding out his sword, Sting.
Averted in The Two Towers: The Rohirrim cavalry charges down a steep incline where orc pikemen are massing. However, the sun rises just in time for them to be dazzled, and the charge completely breaks the line apart.
In the final battle of Pacific Rim, Raiju, a Lightning BruiserKaiju who has already ripped off one of Gipsy Danger's arms, charges at the crippled Jaeger with incredible speed. Gipsy, however, deploys the sword in its other arm and pierces it out just before Raiju hits it. The results are gory... and glorious.
In 10,000 BC, this is how D'Leh kills the mammoth. When his spear gets stuck in the ground, he runs away, leading the beast to impale itself. But in this case D'Leh feels bad because he didn't really kill the mammoth himself, as the one who could kill it would get the White Spear, and more importantly, would be allowed to marry Evolet. He also does it to the terrorbirds, this time intentional.
In Snowpiercer, the tall bearded guy is pierced by a metal rod that Yona puts in his way.
At the climax of the Warrior Cats book Sunset, Brambleclaw is fighting his evil half-brother, Hawkfrost. He'd just saved his leader from a fox trap by digging up the stake holding it in the ground, so he picks up the stake in his mouth and swings it around. Hawkfrost lunges at him and impales himself on the spike. Brambleclaw is shocked and gasps "Hawkfrost! I... I didn't want this."
In Blood Brothers of Gor a Fantasy Counterpart Culture Plains Indians war between two tribes uses this. The "good" tribe sets up a series of battlements, each taller than the last but none of them too terribly impregnable. The "bad" tribe jumps over them on their horses, only to fall into a pit of sharpened sticks where they're impaled.
In another Gor book, Tarl kills a Larl (basically a lion or tiger the size of a small elephant) by getting it to charge at him while he's holding a spear with the butt firmly stuck in the ground.
This is how Sam is able to seriously wound Shelob in The Lord of the Rings. The book explicitly states that he hadn't the strength to pierce her hide, but her forcibly falling on him did.
Andre Norton's Judgment on Janus. After Niall/Ayyar falls into a kalcrok's trap, the kalcrok jumps at him to try to pin him to the wall. It is impaled on his sword, (which he happened to be holding in front of him), killing it.
One episode of The Twilight Zone is "The Once And Future King," wherein Gary Pitkin, an Elvis impersonator, gets transported to 1953, where he meets the real Elvis Presley. At first, Elvis thinks Gary is his stillborn brother Jesse, Back from the Dead. However, when Gary begins coaching Elvis about his music, Elvis is reviled. The two men begin to fight, breaking a guitar at the neck. Then Elvis lunges at Gary; Gary rolls aside, and Elvis impales himself fatally on the jagged guitar neck.
Star Trek: The Original Series first pilot episode "The Menagerie". During the illusionary battle between Captain Pike and a Rigelian warrior, Pike is kneeling in a courtyard holding up a broken spearhead braced against the ground. The warrior jumps down on him and impales himself on the spearhead. Watch it here.
Friday The 13th: The Series episode "Night Prey". A vampire hunter attacks a vampire but the vampire knocks him down. The vampire then leaps onto the man but is impaled on a crucifix the man is holding up, killing it. Watch it here.
Early editions of Dungeons & Dragons: If a PC set a long weapon (such as a spear) in preparation for an opponent's charge and succeeded in hitting the charging opponent, they would do double normal weapon damage, possibly killing the opponent.
Chaosium's ElfQuest RPG. Characters with long thrusting weapons (such as spears) can set them to receive the impact from a charge.
Earthdawn Combat chapter. One of the techniques in the Mounted Combat section is "Setting Against A Charge". If a character knows a Charging Attack is coming and has a spear or lance available, he can make a Melee Weapons Test. If he succeeds, he hits the charging opponent and does damage before the opponent can attack. If he succeeds really well he can knock the opponent off his steed.
Warcraft III. Orc buildings can be outfitted with spikes that damage melee attackers, implied to be this trope. Some units (Crypt Lords, turtles) have an ability that does the same (and in the Crypt Lord's case, gives it extra armor).
In the animation of Brian Jacques' book Martin the Warrior, Martin's sword falls from Badrang's paws and into the prison pit. Martin dives for it and holds it up, and when Badrang leaps into the pit after him, he gets impaled on it.
Parodied & Downplayed example: On The Simpsons when Bart & Lisa get into a fight. Bart is leaving Lisa's room.
Bart: OK, but on my way, I'm going to be doing this: (windmills arms) If you get hit, it's your own fault.
Lisa: OK, then I'm going to start kicking air like this. (kicks) And if any part of you should fill that air, it's your own fault.
(they walk towards each other, then start fighting)
Marge:(in the kitchen) Oh, I better go check that out. Now Homer, don't you eat this pie!
Homer: OK...(Marge leaves) All right, pie, I'm just going to do this. (chomps air) And if you get eaten, it's your own fault! (walks towards pie, chomping air, and hits head on range hood) Ow! Oh, my — aw, to hell with this. (grabs pie, eats it)
Samurai Jack. Several enemies are defeated this way, notably in "Jack and the Spartans" (see phalanx formation in Real Life) and one where a charging elephant robot keeps going after running over Jack holding his sword in the air and its guts spill out from its belly.
This is how boar spears work, as the tactic for boar hunting involves getting a rampaging boar to charge in the direction of the hunter and impale itself on the spear.
Porcupines, upon being threatened, will roll up and stick up its thorns, harming anything that come to harm it. Same happens with porcupine puffer fishes.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.